This invention relates to a key telephone system to be connected to a predetermined number of outside lines, which may either be subscriber lines connected to a central office or station lines connected to a private branch exchange.
A key telephone system generally comprises a key service unit, a plurality of key telephone sets, and a cable serving as an intercom line or path between the key service unit and each telephone set. The key service unit is usually connected to several outside lines. Each telephone set customarily comprises lamps for visually indicating various status, such as idle, calling, held, and busy status, of the respective outside lines. Use is made also of at least one more lamp for intercom calls. Each lamp, as called herein, may be a light emitting diode.
In a conventional key telephone system, each cable comprises a number of core conductors as mentioned to in, for example, U.S. Pat. No. 3,725,601 because each cable has to comprise, besides conductors or leads connected to the respective outside lines through the key service unit, various control and lamp leads for allowing any one of the telephone sets to select one of the outside lines, place an outside call present on an outside line on hold, and lighting a pertinent lamp in every telephone set according to the status of the respective outside lines. This has raised the costs of manufacture and installation of the key telephone system.